Does supplicating against the oppressor with the words: "Allah is sufficient for us, and how excellent an Agent is He. O Allah, take revenge for us against him, and enable us to overcome him, and make him taste what he made us taste," count as a victory, such that the supplicator does not receive the reward for patience, while the oppression might even increase?

What Imam Ahmad mentioned is correct and in accordance with the Sunnah. Avenging oneself against an oppressor has stages. If the oppressed seeks retribution for himself and does to the oppressor what was done to him, it is permissible for him to do so, provided he does not transgress. If he supplicates against him, that is a form of retribution because it alleviates his distress. However, if he leaves his matter to Allah, that is more preferable and better, and it falls under the category of forgiveness. This is indicated by the hadith of Aisha (may Allah be pleased with her) when the Prophet (peace be upon him) supplicated against the one who stole her cloak.

It is not a condition for injustice to be removed by supplication, but supplicating against an oppressor is permissible and there is no harm in it. Allah Almighty says: "Allah does not like the public utterance of evil, except by one who has been wronged. And Allah is All-Hearing, All-Knowing." The Companions also supplicated against those who wronged them.

It has been established in the Prophetic Sunnah that there are supplications that include seeking victory over oppressors and taking revenge on them, but forgiving the oppressor is better and more preferable, as indicated by the Holy Quran and the Prophetic Sunnah.